Soybeans are a major cash crop and investment commodity in North America and elsewhere. Soybean oil is one of the most widely used edible oils, and soybeans are used worldwide both in animal feed and in human food production.
The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) causes substantial yield loss in North American soybean [Glycine max (L.) Merr.] (Mulrooney 1988). Heterodera glycines Ichinohe, was first identified on soybeans in the United States in 1954 at Castle Hayne, N.C. Winstead, et al., Plant Dis. Rep. 39:9-11, 1955. Since its discovery the soybean cyst nematode ("SCN") has been recognized as one of the most destructive pests in soybean. It has been reported in nearly all states in which soybeans are grown, and it causes major production problems in several states, being particularly destructive in the Midwestern states. See generally: Calwell, et al., Agron. J. 52:635-636, 1960; Rao-Arelli and Anand, Crop. Sci. 28:650-652, 1988; Baltazar and Mansur, Soybean Genet. Newsl. 19:120-122, 1992; Concibido, et al., Crop. Sci., 1993. For example, susceptible soybean cultivars had 6-36% lower seed yields than did resistant cultivars on SCN race-3 infested sites in Iowa (Niblack and Norton 1992).
Although the use of nematocides is effective in reducing the population level of the nematode, nematocide use is both uneconomical and potentially environmentally unsound as a control measure in soybean production. Neither is crop rotation a practical means of nematode control, since rotation with a nonsusceptible crop for at least two years is necessary for reducing soybean losses. Therefore, it has long been felt by soybean breeders, that use of resistant varieties is the most practical control measure.
Screening of soybean germplasm for resistance to SCN was begun soon after the discovery of the nematode in the United States, and Golden, et al. (Plant Dis. Rep. 54:544-546, 1970) have described the determination of SCN races. Although SCN was discovered in North America about 40 years ago, soybean breeding for resistance to SCN has mostly utilized genes from two plant introductions--Peking and PI88788, and while these lines have resistance genes for several SCN races, including race-3, they do not provide resistance to all known races.
The plant introduction PI 437.654 is the only known soybean to have resistance to SCN races-3 (Anand 1984), 1, 2, 5, 14 (Anand 1985), 6, and 9 (Rao-Arelli et al. 1992b). However, PI 437.654 has a black seed coat, poor standability, seed shattering, and low yield, necessitating the introgression of its SCN resistance into elite germplasm with a minimum of linkage drag. Conventional breeding with PI 437.654 produced the variety `Hartwig` (Anand 1991), which is more adapted to cultivation and can be used as an alternative source of SCN resistance in soybean breeding programs.
Resistance to SCN is multigenic and quantitative in soybean (Mansur et al. 1993), though complete resistance can be scored qualitatively. For complete resistance to SCN, PI 437.654 has two or three loci for race-3, two or four loci for race-5, and three or four loci for race-14 (Myers and Anand 1991). The multiple genes and SCN races involved contribute to the difficulty breeders have in developing SCN resistant soybean varieties.
Breeding programs for SCN resistance rely primarily on field evaluations where natural nematode populations occur. However, these populations can be mixtures of undetermined races (Young 1982) and the environment can affect the overwintering and infection capability of the nematodes (Niblack and Norton 1992). Although evaluations using inbred nematode populations in controlled greenhouse environments are superior, they are prohibitively expensive and the nematodes are difficult to manage for large breeding programs (Rao-Arelli, pers comm). These deficiencies in each evaluation method make SCN resistance a difficult trait to manipulate in soybean improvement programs.
Genetic markers closely linked to important genes may be used to indirectly select for favorable alleles more efficiently than direct phenotypic selection (Lande and Thompson 1990). The i allele at the I locus is responsible for black or imperfect black seed-coat type, and is a morphological genetic-marker closely linked in coupling to the SCN resistance allele, Rhg.sub.4, in the variety Peking (Matson and Williams 1965). The I locus is mapped to linkage group VII of the classical genetic map (Weiss 1970) and to linkage group A of a public RFLP map (Keim et al. 1990). SCN race-3 resistance loci are also associated with RFLP markers mapped to linkage groups A, G and K in the soybean PI 209.332 (Concibido et al. 1994).
Therefore, it is of particular importance, both to the soybean breeder and to farmers who grow and sell soybeans as a cash crop, to identify, through genetic mapping, the quantitative trait loci (QTL) for resistance to the various SCN races. Knowing the QTLs associated with resistance to the SCN races, soybean breeders will be better able to breed SCN resistant soybeans which also possess the other genotypic and phenotypic characteristics required for commercial soybean lines.